Insulation components are used in high voltage electrical equipment, such as for example capacitors, transformers or switchgear barriers. Dielectrics of the insulation component can, however, exhibit regions of high electric field intensity which may cause partial discharges that are characterized in that they do not completely bridge the electrodes.
The partial discharges quite often start, for example, within inhomogeneities such as voids or cracks within a solid insulation component or bubbles within a liquid insulation component. For simplicity of presentation the inhomogeneities are referred to herein as “voids”. The voids can be gas-filled. If the voltage stress across a void exceeds the inception voltage for the gas within the void, the gas ionizes and partial discharges can start to occur within the void. The partial discharges can then cause progressive deterioration of the material of the insulation component which ultimately might lead to an electrical breakdown of the insulation component.
To avoid deterioration and electrical breakdown in the field and to control the quality of an insulation component, an insulation component can be tested for voids by a partial discharge test before leaving the factory. Such a known partial discharge test can include applications of a high AC overvoltage for a limited duration on the order of a minute combined with partial discharge measurements (in the following called the known partial discharge test).
For a partial discharge to occur—in addition to the voltage stress across the void exceeding the inception voltage—there should be enough free electrons within or at the void to initiate an electron avalanche within the void and hence a partial discharge. A free electron may also be called start electron. In a virgin insulation component the most likely event to cause such a free electron is background radiation. Another mechanism for the creation of start electrons is field emission from the void surface.
This means, that even in the case of voids being present, they may not be detected due to a lack of free electrons in or at the void or due to a statistical time lag, being the time required for a free electron to appear in the void. The time for such a free electron to appear within a void can increase with a decrease in void size. In particular, small voids may not develop partial discharge activity within the duration of the known partial discharge test and therefore may not be detected. Known partial discharge tests often use very high AC voltages, which leads to field emission from the void surface, to force also the small voids into discharge. However, using such high AC voltage may cause irreversible damage by initiating electrical trees from other stressed enhancing defects, such as inclusions of metallic particles which might have been harmless at operating stress.
To provide the initiatory free electrons it is known to expose the insulation component to ionizing radiation, e.g. X-ray irradiation, concurrently to the application of the AC voltage and the measuring of the partial discharge activity. The exposure to a continuous X-ray beam has been described in “Partial Discharge—Part XV: Improved PD Testing of Solid Dielectrics using X-ray Induced Discharge Initiation”, N. Fujimoto et al., IEEE Electrical Insulation Magazine, Vol. 8, No. 6, 1992, pp. 33-41, “Modulation of Partial Discharge Activity in GIS Insulators by X-ray Irradiation” by J. M. Braun et al., IEEE Transactions on Electrical Insulation, Vol. 26, No. 3, June 1991, pp. 460-468, “X-ray Induced Partial Discharge—an Application for High Voltage Insulation Diagnostics” by L. S. Pritchard et al., Proceedings of the IEE Colloquium on Materials Characterisation—How Can We Do It? What Can It Tell Us? (Ref. No: 1997/150), December 1997, pp. 7/1-7/3, “Location of Partial Discharges in High Voltage Equipment Using Ionizing Rays” by J. Svitek, Proceedings of the 5th International Conference on Dielectric Materials, Measurements and Applications, June 1988, pp. 183-186). With the provision of free electrons by X-ray irradiation, the time lag/delay before partial discharge initiation can be reduced. Furthermore, the inception voltage can be reduced to a value which is considered to be much closer to the true value inherent to the properties of the insulation component and the defect type (i.e. void type).
Further, it has been proposed to expose the insulation component to a pulsed X-ray irradiation (“Study of Continuous and Pulse X-ray Induced Partial Discharge Statistical Behaviour in Epoxy Samples” by G. C. da Silva et al., Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials, June 2003, Session S7-1, pp. 831-834, “Continuous and Pulsed X-ray Induced Partial Discharges: Similarities and Differences” by G. S. Silva at al., 2006 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp. 598-601). In this case, an X-ray tube is used for generating a continuous X-ray beam. For the generation of pulsed X-ray beams a chopper is inserted between the X-ray tube and the insulation component. The chopper includes a lead disk with two rectangular windows operated by an AC motor. The length/duration of the such obtained X-ray pulses is approximately 2 ms per cycle. With a combination of an X-ray tube with a chopper, X-ray pulses with a shorter pulse length can generally not be obtained.
When pulsed X-ray irradiation is used, the partial discharges occur only during the interval of X-ray pulse application. Hence, the partial discharges are modulated by the X-ray pulses.
With the application of continuous X-ray irradiation or pulsed X-ray irradiation with a pulse length of 2 ms or higher the magnitude of the induced partial discharge pulses may, however, be considerably lowered, such that a precision, low noise partial discharge detection equipment may be required for detection of the partial discharge pulses. Such high precision, low noise partial discharge detection equipment is, however, not suitable for usage in a factory environment.